Add cmdline args to plugin cmd

[mostafa]

This PR adds the ability to pass cmdline args to the plugin via the args key of the plugin listing in the gatewayd_plugins.yaml config file. So a possible plugin config file will like this:

gatewayd-plugin-test:
  # whether to enable or disable the plugin on the next run
  enabled: True
  # path to the plugin's binary file
  localPath: ../gatewayd-plugin-test/gatewayd-plugin-test
  # Pass cmdline args to the plugin
  args: ["--log-level", "info"]
  # Checksum hash to verify the binary before loading
  checksum: a20e3a1529f4460ad24416a055d599f6f55f42ead836345a4bf0a17d056abb45

Fixes #72.